High-efficiency modulating RF amplifier

ABSTRACT

The present invention, generally speaking, provides for high-efficiency power control of a high-efficiency (e.g., hard-limiting or switch-mode) power amplifier in such a manner as to achieve a desired modulation. In one embodiment, the spread between a maximum frequency of the desired modulation and the operating frequency of a switch-mode DC-DC converter is reduced by following the switch-mode converter with an active linear regulator. The linear regulator is designed so as to control the operating voltage of the power amplifier with sufficient bandwidth to faithfully reproduce the desired amplitude modulation wave-form. The linear regulator is further designed to reject variations on its input voltage even while the output voltage is changed in response to an applied control signal. This rejection will occur even though the variations on the input voltage are of commensurate or even lower frequency than that of the controlled output variation. Amplitude modulation may be achieved by directly or effectively varying the operating voltage on the power amplifier while simultaneously achieving high efficiency in the conversion of primary DC power to the amplitude modulated output signal. High efficiency is enhanced by allowing the switch-mode DC-to-DC converter to also vary its output voltage such that the voltage drop across the linear regulator is kept at a low and relatively constant level. Time-division multiple access (TDMA) bursting capability may be combined with efficient amplitude modulation, with control of these functions being combined. In addition, the variation of average output power level in accordance with commands from a communications system may also be combined within the same structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to RF amplifiers and signal modulation.

2. State of the Art

Battery life is a significant concern in wireless communications devicessuch as cellular telephones, pagers, wireless modems, etc.Radio-frequency transmission, especially, consumes considerable power. Acontributing factor to such power consumption is inefficient poweramplifier operation. A typical RF power amplifier for wirelesscommunications operates with only about 10% efficiency. Clearly, alow-cost technique for significantly boosting amplifier efficiency wouldsatisfy an acute need.

Furthermore, most modern digital wireless communications devices operateon a packet basis. That is, the transmitted information is sent in aseries of one or more short bursts, where the transmitter is active onlyduring the burst times and inactive at all other times. It is thereforealso desirable that control of burst activation and deactivation becontrolled in an energy-efficient manner, further contributing toextended battery life.

Power amplifiers are classified into different groups: Class A, Class B,Class AB, etc. The different classes of power amplifiers usually signifydifferent biasing conditions. In designing an RF power amplifier, thereis usually a trade-off between linearity and efficiency. The differentclasses of amplifier operation offer designers ways to balance these twoparameters.

Generally speaking, power amplifiers are divided into two differentcategories, linear and non-linear. Linear amplifiers (e.g. Class Aamplifiers and Class B push-pull amplifiers), maintain high linearity,resulting in faithful reproduction of the input signal at their outputsince the output signal is linearly proportional to the input signal. Innon-linear amplifiers (e.g. single-ended Class B and Class Camplifiers), the output signal is not directly proportional to the inputsignal. The resulting amplitude distortion on the output signal makesthese amplifiers most applicable to signals without any amplitudemodulation, which are also known as constant-envelope signals.

Amplifier output efficiency is defined as the ratio between the RFoutput power and the input (DC) power. A major source of power amplifierinefficiency is power dissipated in the transistor. A Class A amplifieris inefficient since current flows continuously through the device.Conventionally, efficiency is improved by trading-off linearity forincreased efficiency. In Class B amplifiers, for example, biasingconditions are chosen such that the output signal is cut off during halfof the cycle unless the opposing half is provided by a second transistor(push-pull). As a result, the waveform will be less linear. The outputwaveform may still be made sinusoidal using a tank circuit or otherfilter to filter out higher and lower frequency components.

Class C amplifiers conduct during less than 50% of the cycle, in orderto further increase efficiency; i.e., if the output current conductionangle is less than 180 degrees, the amplifier is referred to as Class C.This mode of operation can have a greater efficiency than Class A orClass B, but it typically creates more distortion than Class A or ClassB amplifiers. In the case of a Class C amplifier, there is still somechange in output amplitude when the input amplitude is varied. This isbecause the Class C amplifier operates as a constant currentsource-albeit one that is only on briefly-and not a switch.

The remaining classes of amplifiers vigorously attack the problem ofpower dissipation within the transistor, using the transistor merely asa switch. The underlying principle of such amplifiers is that a switchideally dissipates no power, for there is either zero voltage across itor zero current through it. Since the switch's V-I product is thereforealways zero, there is no dissipation in this device. A Class E poweramplifier uses a single transistor, in contrast with a Class D poweramplifier, which uses two transistors

In real life, however, switches are not ideal. (Switches have turnon/off time and on-resistance.) The associated dissipation degradesefficiency. The prior art has therefore sought for ways to modifyso-called “switch-mode” amplifiers (in which the transistor is driven toact as a switch at the operating frequency to minimize the powerdissipated while the transistor is conducting current) so that theswitch voltage is zero for a non-zero interval of time about the instantof switching, thereby decreasing power dissipation. The Class Eamplifier uses a reactive output network that provides enough degrees offreedom to shape the switch voltage to have both zero value and zeroslope at switch turn-on, thus reducing switching losses. Class Famplifiers are still a further class of switch-mode amplifiers. Class Famplifiers generate a more square output waveform as compared to theusual sinewave. This “squaring-up” of the output waveform is achieved byencouraging the generation of odd-order harmonics (i.e., x3, x5, x7,etc.) and suppressing the even-order harmonics (i.e., x2, x4, etc.) inthe output network.

An example of a known power amplifier for use in a cellular telephone isshown in FIG. 1. GSM cellular telephones, for example, must be capableof programming output power over a 30 dBm range. In addition, thetransmitter turn-on and turn-off profiles must be accurately controlledto prevent spurious emissions. Power is controlled directly by the DSP(digital signal processor) of the cellular telephone, via a DAC (digitalto analog converter). In the circuit of FIG. 1, a signal GCTL drives thegate of an external AGC amplifier that controls the RF level to thepower amplifier. A portion of the output is fed back, via a directionalcoupler, for closed-loop operation. The amplifier in FIG. 1 is not aswitch-mode amplifier. Rather, the amplifier is at best a Class ABamplifier driven into saturation, and hence demonstrates relatively poorefficiency.

Survey of Prior Patents

Control of the output power from an amplifier is consistently shown asrequiring a feedback structure, as exemplified in U.S. Pat. Nos.4,392,245; 4,992,753; 5,095,542; 5,193,223; 5,369,789; 5,410,272;5,697,072 and 5,697,074. Other references, such as U.S. Pat. No.5,276,912, teach the control of amplifier output power by changing theamplifier load circuit.

A related problem is the generation of modulated signals, e.g.,amplitude modulated (AM) signals, quadrature amplitude modulated signals(QAM), etc. A known IQ modulation structure is shown in FIG. 2. A datasignal is applied to a quadrature modulation encoder that produces I andQ signals. The I and Q signals are applied to a quadrature modulatoralong with a carrier signal. The carrier signal is generated by acarrier generation block to which a tuning signal is applied.

Typically, an output signal of the quadrature modulator is then appliedto a variable attenuator controlled in accordance with a power controlsignal. In other instances, power control is implemented by vaying thegain of the amplifier. This is achieved by adjusting the bias ontransistors within the inear amplifier, taking advantage of the effectwhere transistor transconductance varies with the aplied biasconditions. Since amplifier gain is strongly related to the transistortransconductance, varying the transconductance effectively varies theamplifier gain. A resulting signal is then amplified by a linear poweramplifier and applied to an antenna.

A method for producing accurate amplitude modulated signals usingnonlinear Class C amplifiers, called “plate modulation,” has been knownfor over 70 years as described in texts such as Terman's Radio EngineersHandbook (McGraw-Hill, 1943). In the typical plate-modulation technique,output current from the modulator amplifier is linearly added to thepower supply current to the amplifying element (vacuum tube ortransistor), such that the power supply current is increased anddecreased from its average value in accordance with the amplitudemodulation. This varying current causes the apparent power supplyvoltage on the amplifying element to vary, in accordance with theresistance (or conductance) characteristics of the amplifying element.

By using this direct control of output power, AM can be effected as longas the bandwidth of the varying operating voltage is sufficient. Thatis, these nonlinear amplifiers actually act as linear amplifiers withrespect to the amplifier operating voltage. To the extent that thisoperating voltage can be varied with time while driving the nonlinearpower amplifier, the output signal will be linearly amplitude modulated.

In AM signals, the amplitude of the signal is made substantiallyproportional to the magnitude of an information signal, such as voice.Information signals such as voice are not constant in nature, and so theresulting AM signals are continuously varying in output power. Methodsof achieving amplitude modulation include the combination of a multitudeof constant amplitude signals, as shown in U.S. Pat. Nos. 4,580,111;4,804,931; 5,268,658 and 5,652,546. Amplitude modulation by usingpulse-width modulation to vary the power supply of the power amplifieris shown in U.S. Pat. Nos. 4,896,372; 3,506,920; 3,588,744 and3,413,570. However, the foregoing patents teach that the operatingfrequency of the switch-mode DC-DC converter must be significantlyhigher than the maximum modulation frequency.

U.S. Pat. No. 5,126,688 to Nakanishi et al. addresses the control oflinear amplifiers using feedback control to set the actual amplifieroutput power, combined with periodic adjustment of the power amplifieroperating voltage to improve the operating efficiency of the poweramplifier. The primary drawback of this technique is the requirement foran additional control circuit to sense the desired output power, todecide whether (or not) the power amplifier operating voltage should bechanged to improve efficiency, and to effect any change if so decided.This additional control circuitry increases amplifier complexity anddraws additional power beyond that of the amplifier itself, whichdirectly reduces overall efficiency.

A further challenge has been to generate a high-power RF signal havingdesired modulation characteristics. This object is achieved inaccordance with the teachings of U.S. Pat. No. 4,580,111 to Swanson byusing a multitude of high efficiency amplifiers providing a fixed outputpower, which are enabled in sequence such that the desired totalcombined output power is a multiple of this fixed individual amplifierpower. In this scheme, the smallest change in overall output power isessentially equal to the power of each of the multitude of highefficiency amplifiers. If finely graded output power resolution isrequired, then potentially a very large number of individual highefficiency amplifiers may be required. This clearly increases theoverall complexity of the amplifier.

U.S. Pat. No. 5,321,799 performs polar modulation, but is restricted tofull-response data signals and is not useful with high power,high-efficiency amplifiers. The patent teaches that amplitude variationson the modulated signal are applied through a digital multiplierfollowing phase modulation and signal generation stages. The finalanalog signal is then developed using a digital-to-analog converter. Asstated in the State of the Art section herein, signals with informationalready implemented in amplitude variations are not compatible withhigh-efficiency, nonlinear power amplifiers due to the possibly severedistortion of the signal amplitude variations.

Despite the teachings of the foregoing references, a number of problemsremain to be solved, including the following: to achieve high-efficiencyamplitude modulation of an RF signal by varation of the operatingvoltage using a switch mode converter without requiring high-frequencyswitch-mode operation (as compared to the modulation frequency); tounify power-level and burst control with modulation control; to enablehigh-efficiency modulation of any desired character (amplitude and/orphase); and to enable high-power operation (e.g., for base stations)without sacrificing power efficiency.

SUMMARY OF THE INVENTION

The present invention, generally speaking, provides for high-efficiencypower control of a high-efficiency (e.g., hard-limiting or switch-mode)power amplifier in such a manner as to achieve a desired modulation. Inone embodiment, the spread between a maximum frequency of the desiredmodulation and the operating frequency of a switch-mode DC-DC converteris reduced by following the switch-mode converter with an active linearregulator. The linear regulator is designed so as to control theoperating voltage of the power amplifier with sufficient bandwidth tofaithfully reproduce the desired amplitude modulation wave-form. Thelinear regulator is further designed to reject variations on its inputvoltage even while the output voltage is changed in response to anapplied control signal. This rejection will occur even though thevariations on the input voltage are of commensurate or even lowerfrequency than that of the controlled output variation. Amplitudemodulation may be achieved by directly or effectively varying theoperating voltage on the power amplifier while simultaneously achievinghigh efficiency in the conversion of primary DC power to the amplitudemodulated output signal. High efficiency is enhanced by allowing theswitch-mode DC-to-DC converter to also vary its output voltage such thatthe voltage drop across the linear regulator is kept at a low andrelatively constant level. Time-division multiple access (TDMA) burstingcapability may be combined with efficient amplitude modulation, withcontrol of these functions being combined. In addition, the variation ofaverage output power level in accordance with commands from acommunications system may also be combined within the same structure.

The high-efficiency amplitude modulation structure may be extended toany arbitrary modulation. Modulation is performed in polar form, i.e.,in a quadrature-free manner.

Single high-efficiency stages may be combined together to formhigh-power, high-efficiency modulation structures.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be further understood from the followingdescription in conjunction with the appended drawing. In the drawing:

FIG. 1 is a block diagram of a known power amplifier with output powercontrolled by varying the power supply voltage;

FIG. 2 is a block diagram of a known IQ modulation structure;

FIG. 3 is a block diagram of a power amplifier in accordance with anexemplary embodiment of the present invention;

FIG. 4 is a plot comparing saturated Class AB power amplifier outputpower versus operating voltage with the mathematical model V=√{squareroot over (PR)};

FIG. 5 is a waveform diagram illustrating operation of one embodiment ofthe invention;

FIG. 6 is a waveform diagram illustrating operation of anotherembodiment of the invention;

FIG. 7 is a waveform diagram illustrating bursted AM operation;

FIG. 8 is a waveform diagram illustrating bursted AM operation withpower level control;

FIG. 9 is a block diagram of a polar modulation structure using ahigh-efficiency amplifier;

FIG. 10 is a block diagram of a first high power, high efficiency,amplitude modulating RF amplifier;

FIG. 11 is a waveform diagram illustrating operation of the amplifier ofFIG. 10;

FIG. 12 is a block diagram of a second high power, high efficiency,amplitude modulating RF amplifier; and

FIG. 13 is a waveform diagram illustrating operation of the amplifier ofFIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 3, a block diagram is shown of a power amplifierthat may be used in the present invention. A switch-mode (or saturated)nonlinear amplifier has applied to it a voltage produced by a powercontrol stage. In an exemplary embodiment, the voltage V applied to thenonlinear amplifier is controlled substantially in accordance with theequationV=√{square root over (PR)}where P is the desired power output level of the amplifier and R is theresistance of the amplifier. In the case of a switch-mode or saturatedamplifier, the resistance R may be regarded as constant. The powercontrol stage receives a DC input voltage, e.g., from a battery, andreceives a power level control signal and outputs a voltage inaccordance with the foregoing equation.

The efficacy of directly controlling output power of nonlinearamplifiers over a wide dynamic range by solely varying the operatingvoltage is demonstrated by FIG. 4, showing a plot comparing saturatedClass AB power amplifier output power versus operating voltage with themathematical model V=√{square root over (PR)}.

Referring again to FIG. 3, a power control circuit in accordance with anexemplary embodiment of the invention is shown. A power control stageincludes a switch-mode converter stage and a linear regulator stageconnected in series. The switch-mode converter may be a Class D device,for example, or a switch-mode power supply (SMPS). The switch-modeconverter efficiently steps down the DC voltage to a voltage thatsomewhat exceeds but that approximates the desired power-amplifieroperating voltage level. That is, the switch-mode converter performs anefficient gross power level control. The switch-mode converter may ormay not provide sufficiently fine control to define ramp portions of adesired power envelope.

The linear regulator performs a filtering function on the output of theswitch-mode converter. That is, the linear regulator controls precisepower-envelope modulation during a TDMA burst, for example. The linearregulator may or may not provide level control capabilities like thoseof the switch-mode converter.

Note that, depending on the speed of the switch-mode converter and thelinear regulator, the power control stage may be used to perform powercontrol and/or amplitude modulation. A control signal PL/BURST is inputto a control block, which outputs appropriate analog or digital controlsignals for the switch-mode converter and the linear regulator. Thecontrol block may be realized as a ROM (read-only memory) and/or a DAC(digital to analog converter).

Referring to FIG. 5, a waveform diagram is shown, illustrating operationof one embodiment of the invention. The waveforms A and B representanalog control signals applied to the switch-mode converter and to thelinear regulator, respectively. The waveforms V₁ and V₂ represent theoutput voltages of the switch-mode converter and to the linearregulator, respectively. Assume that the switch-mode converter has arelatively large time constant, i.e., that it ramps relatively slowly.When the control signal A is set to a first non-zero power level, thevoltage V₁ will then begin to ramp toward a commensurate voltage.Because of the switch-mode nature of the converter, the voltage V₁ mayhave a considerable amount of ripple. An amount of time required toreach that voltage defines the wakeup period. When that voltage isreached, the control signal B is raised and lowered to define a seriesof transmission bursts. When the control signal B is raised, the voltageV₂ ramps quickly up to a commensurate voltage, and when the controlsignal B is lowered, the voltage V₂ ramps quickly down. Following aseries of bursts (in this example), the control signal A is raised inorder to increase the RF power level of subsequent bursts. The controlsignal B remains low during a wait time. When the voltage V₁ has reachedthe specified level, the control signal B is then raised and lowered todefine a further series of transmission bursts.

The voltage V₂ is shown in dotted lines superimposed on the voltage V₁.Note that the voltage V₂ is less than the voltage V₁ by a small amount,greater than the negative peak ripple on the voltage V₁ . This smalldifference between the input voltage of the linear regulator V₁ and theoutput voltage of the linear regulator V₂ makes overall high-efficiencyoperation possible.

Referring to FIG. 6, in accordance with a different embodiment of theinvention, the switch-mode converter is assumed to have a relativelyshort time constant; i.e., it ramps relatively quickly. Hence, when thecontrol signal A is raised, the voltage V₁ ramps quickly to thecommensurate voltage. The control signal B is then raised, and thevoltage V₂ is ramped. The time difference between when the controlsignal A is raised on the control signal B is raised defines the wake uptime, which may be very short, maximizing sleep time and power savings.The control signal B is then lowered at the conclusion of thetransmission burst, after which the control signal A is lowered.Following the example of FIG. 5, in FIG. 6, when the control signal A isnext raised, it defines a higher power level. Again, the voltage V₂ issuperimposed in dotted lines on the voltage V₁ .

The same structure may be used to perform amplitude modulation inaddition to power and burst control. Referring to FIG. 7, a waveformdiagram is shown illustrating bursted AM operation. An output signal ofthe switch-mode converted is shown as a solid line. As a burst begins,the output signal of the switch-mode converter ramps up. Optionally, asshown in dashed line, the switch-mode converter may ramp up to a fixedlevel with the linear regulator effecting all of the amplitudemodulation on the output signal. More preferably, from an. efficiencystandpoint, the switch-mode converter effects amplitude modulation,producing an output signal that, ignoring noise, is a small fixed offsetΔV above the desired output signal. The linear regulator removes thenoise from the output signal of the switch-mode converter, effectivelyknocking down the signal by the amount ΔV. The output signal of thelinear regulator is shown as a dotted line in FIG. 7. At the conclusionof the burst, the signals ramp down.

Full control of the output signal power level (average power of thesignal) is retained. A succeeding burst, for example, might occur at ahigher power level, as shown in FIG. 8. As compared to FIG. 7, in FIG.8, all signals scale appropriately to realized a higher average poweroutput.

Incorporation of amplitude modulation on a phase-modulated signal,though it complicates the signal generation method, is often desirablesince such signals may, and often do, occupy less bandwidth than purelyphase-modulated signals. Referring to FIG. 9, a block diagram is shownof a polar modulation structure using a high-efficiency amplifier of thetype described thus far. This polar modulation structure is capable ofeffecting any desired modulation. A data signal is applied to amodulation encoder that produces magitude and phase signals. The phasesignal is applied to a phase-modulation-capable carrier generationblock, to which a tuning signal is also applied. A resulting signal isthe amplified by a nonlinear power amplifier of the type previouslydescribed. Meanwhile, the magnitude signal is applied to a magnitudedriver. The magnitude driver also receives a power control signal. Inresponse, the magnitude driver produces an operating voltage that isapplied to the non-linear amplifier. The magnitude driver and thenon-linear amplifier may be realized in the same manner as FIG. 3,described previously, as indicated in FIG. 9 by a dashed line.

The modulation structures described thus far are suitable for use in,among other applications, cellular telephone handsets. A similar needfor high-efficiency RF signal generation exists in cellular telephonebasestations. Basestations, however, operate at much higher power thanhandsets. The following structure may be used to achieve high-power,high-efficiency RF signal generation.

Referring to FIG. 10, a first high power, high efficiency, amplitudemodulating RF amplifier includes multiple switch mode power amplifier(SMPA) blocks, each block being realized as shown in FIG. 3, forexample. An RF signal to be amplified is input to all of the SMPA blocksin common. Separate control signals for each of the SMPA blocks aregenerated by a magnitude driver in response to a magnitude input signal.Output signals of the SMPA blocks are summed to form a single resultantoutput signal.

The manner of operation of the amplifier of FIG. 10 may be understoodwith reference to FIG. 11. On the left-hand side is shown an overallmagnitude signal that is applied to the magnitude driver. On theright-hand side are shown SMPA drive signals output by the magnitudedriver to be applied to the respective SMPAs. Note that the sum of theindividual drive signals yields the overall magnitude signal.

An alternative embodiment of a high-power amplifier is shown in FIG. 12.In this embodiment, instead of generating individual drive signals forthe respective SMPAs, a common drive signal is generated and applied incommon to all of the SMPAs. At a given instant in time, the common drivesignal is caused to have a value that is one Nth of an overall magnitudesignal applied to the magnitude driver, where N is the number of SMPAs.The result is illustrated in FIG. 13. Once again, note that the sum ofthe individual drive signals yields the overall magnitude signal.

It will be appreciated by those of ordinary skill in the art that theinvention can be embodied in other specific forms without departing fromthe spirit or essential character thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restrictive. The scope of the invention is indicated by theappended claims rather than the foregoing description, and all changeswhich come within the meaning and range of equivalents thereof areintended to be embraced therein.

1-6. (canceled)
 7. A power amplifier apparatus, comprising: aswitch-mode power amplifier having an RF input port, a power supplyinput port, and an RF output port; a control circuit having a primarycontrol signal input and a first control signal output; and a powerconverter having a first control signal input configured to receive afirst control signal from the first control signal output of the controlcircuit, a DC input configured to receive a DC supply voltage, and apower converter output coupled to the power supply input port of theswitch-mode power amplifier.
 8. The power amplifier apparatus of claim 7wherein the power converter is a switch-mode power converter.
 9. Thepower amplifier apparatus of claim 7 wherein the control circuitcomprises a read-only memory (ROM).
 10. The power amplifier apparatus ofclaim 7 wherein the control circuit comprises a digital-to-analogconverter (DAC).
 11. The power amplifier apparatus of claim 7, furthercomprising a linear regulator coupled between the power converter outputand the power supply input port of the switch-mode power amplifier. 12.The power amplifier apparatus of claim 11 wherein the linear regulatorhas a second control signal input configured to receive a second controlsignal from a second control signal output of the control circuit. 13.The power amplifier apparatus of claim 12 wherein the second controlsignal is conditioned to effect the generation of a series oftransmission bursts.
 14. The power amplifier apparatus of claim 13wherein the first control signal is conditioned to effect the powerlevel of the transmission bursts.
 15. The power amplifier apparatus ofclaim 11 wherein the first control signal is conditioned to cause thepower converter to vary its output voltage in a manner that reduces thevoltage drop across the linear regulator.
 16. The power amplifierapparatus of claim 7 wherein the power converter is configured to effectbursted amplitude modulation.
 17. The power amplifier apparatus of claim16 wherein the RF input port is configured to receive a phase-modulatedRF input signal.
 18. A polar modulation structure, comprising: anon-linear power amplifier having a supply voltage input and a phasecomponent input; a magnitude driver having an input configured toreceive a magnitude component of an input data signal and an outputconfigured to provide a variable supply voltage to the supply voltageinput of the non-linear power amplifier; and a carrier generator havingan input configured to receive a phase component of the input datasignal and an output configured to provide a phase-modulated signal tothe phase component input of the non-linear power amplifier.
 19. Thepolar modulation structure according to claim 18, further comprising amodulation encoder operable to generate the magnitude and phasecomponents of the input data signal.